Upper trunk injury of the brachial plexus predominantly affects which region?

• A. Intrinsic hand muscles
• B. Forearm extensor muscles
• C. Shoulder muscles
• D. Trunk muscles

Answer: (C)

Injury to the upper trunk disrupts the C5–C6 fibers that feed nerves to the shoulder region. These fibers give rise to important nerves for the shoulder, including the suprascapular nerve (which innervates the supraspinatus and infraspinatus) and contribute to the axillary nerve (which innervates the deltoid and teres minor). Loss of these muscles produces weakness in shoulder abduction (supraspinatus initiating, then deltoid) and external rotation (infraspinatus and teres minor). Sensation over the lateral shoulder (regimental badge area) may also be affected via the axillary nerve. Intrinsic hand muscles are mainly supplied by lower trunk fibers (C8–T1) and the forearm extensors by the radial nerve with broader C5–T1 input, so upper trunk injuries don’t predominantly affect those regions. Trunk muscles are supplied by other branches, not primarily by the upper trunk, so the shoulder region is the most impacted.